1. Field of the Invention
This invention relates to ballasts which connect a discharge lamp to a power supply and, more particularly, to an electronic ballast for use with a metal halide lamp.
2. Description Of The Prior Art
High intensity discharge lamps operate by heating a contained gas to a sufficient level that it becomes excited and emits photons. In a typical DC lamp arrangement, a quartz arc tube contains a quantity of gas, such as mercury alone or combined with halides of a metal, such as sodium or scandium. A pair of main electrodes are positioned at opposite ends of the quartz arc tube and a starting electrode may be positioned adjacent one of the main electrodes. Unlike incandescent lamps, high intensity discharge lamps do not become immediately operational when connected to a power supply. These discharge lamps pass through several modes of operation before reaching a normal, steady-state mode of operation. When the power supply is initially connected to the discharge lamp, a high starting voltage is applied between the starting electrode and the adjacent main electrode, or between the main electrodes, and begins to charge the gas therein. This period of operation is generally referred to as a glow mode. Once the gas has been sufficiently charged throughout the tube, the gas will break over and a high current arc will form between the main electrodes. The arc quickly settles to a lower current, normal voltage level in which the arc discharge of the lamp generates a desired light output.
It is common to use an electronic or other ballast to control the operation of a discharge lamp. In general, an acceptable ballast must at least limit the power supplied to the lamp, otherwise the lamp would be subject to an ever increasing current which would destroy the lamp. Electronic ballasts for metal halide lamps may include two converters which generate the required initial high voltage to break over the lamp and a compliance voltage to transition the lamp from the glow mode to the arc mode. See, for example, U.S. Pat. No. 4,749,913. Although this arrangement has been shown to be successful, it lacks a systematic approach which can achieve a low cost, high performance design. The electronic ballast shown in U.S. Pat. No. 4,904,907 applies a regulated square wave of current to a metal halide discharge lamp to control its operation. A power control circuit useable with a discharge lamp is shown in U.S. Pat. No. 4,928,038.
Most electronic ballasts also control the lamp current, but this is not considered a compatible means to ballast a high intensity discharge lamp. As the voltage rises in a discharge lamp, the lamp current is constant, and the lamp power will, therefore, increase with age. This power increase has the adverse effect of lowering the lamp life and increasing the possibility of a non-passive lamp failure. A good power factor is not a characteristic of the conversion process of these known types of ballasts. Therefore, an additional converter, or a passive filter network, is required to correct the power factor to industry standards when used with a high intensity discharge lamp. Typical passive component, metal halide ballasts are economical but lack the performance and compactness to extend their application. Their typical electronic counterparts are often expensive and are not designed around a topology which offers the optimal blend of performance, cost and compactness. Many ballast techniques are not readily compatible with AC and DC metal halide lamps.
Therefore, it is an object of the present invention to provide an electronic ballast which is useful in controlling metal halide and other high intensity gas discharge lamps. It is an object to provide such a ballast in a low cost arrangement which does not require multiple converters for its operation. It is also an object to provide such a ballast in an arrangement which is small and lightweight. It is a further object to provide such a ballast in an arrangement which can accurately control the lamp power and render it compatible with various lamps. Finally, it is an object of the present invention to provide a gas discharge lamp ballast which can operate either DC or AC metal halide lamps with minor modifications.